1. Field of the Invention
The present invention relates in general to nanofabrication and nanometrology methods and to high resolution systems for manufacturing and measuring very small structures in wafers and substrates.
2. Description of the Background Art
High Resolution nanofabrication on a manufacturing wafer is required when manufacturing integrated circuits (ICs) including millions of microscopic circuit elements on the surface of tiny chips of silicon taken from the wafer, which is typically made of Silicon or Polysilicon, and other semiconductor materials. These chips are used to make ICs such as computer processors, memory chips, and many other devices. Nanofabrication has also been used, more recently to manufacture a wide variety of released structures in wafers or substrates which are referred to as mechanical microstructures (“MEMS”).
Wafers are traditionally coated with a reactive photoresist and then exposed to a selected pattern which can be defined in a mask or can be projected onto the wafer without a mask using a laser or electron beam. The photoresist is later etched or processed in a way that permits selective removal of exposed portions of the wafer. During exposure, the wafer or substrate is typically supported within a controlled-environment chamber upon a movable “stage.”
For those systems that do not employ a mask, the precise placement and high resolution of features is determined by controlling the projected beam, and so typically, for very small beam deflections, deflection ‘lenses’ are used, while larger beam deflections require electromagnetic scanning. Because of the inaccuracy and because of the finite number of steps in the exposure grid the writing field is of the order of 100-1000 micrometer (˜1 mm). Larger patterns have required stage moves, and so an accurate stage is critical for stitching (tiling writing fields exactly against each other) and pattern overlay (aligning a pattern to a previously made pattern).
The precision and speed of the nanofabrication process is therefore dependent in part, on the precision of the stage's placement, and the movable stage is made of expensive and precisely controlled structural elements and sensors. The stage must therefore have considerable mass, and the time taken to move that mass and then confirm or adjust the stage's position adds to the time needed for high resolution fabrication for any wafer having a large area.
Complex systems which integrate, for example, beam deflection control and stage movement control necessarily require considerable care in development and operation and those systems can be very expensive to configure and calibrate. Broadly speaking, the problem is determining the position of an object in 3D space with nanometer precision, which is stable over long periods of time. A particular physical example of this problem is that of placing scanning-probe, used in atomic surface microscopy [see e.g., G Binnig, H Rohrer, Ch Gerber, E Weibel, Surface Studies by Scanning Tunneling Microscopy, Phys Rev Lett 49, 57-61 (1982); G Binnig, C F Quate, Ch Gerber, Atomic Force Microscope, Phys Rev Lett 56, 930-933 (1966)] and tip-based micro- and nano-fabrication [A A Tseng, A Notargiacomo, T P Chen, Nanofabrication by scanning probe microscope lithography: a review, J Vac Sci Technol B 23, 877-894 (2005)] and tip-based nano- and micro-fabrication [A A Tseng, A Notargiacomo, T P Chen, Nanofabrication by scanning probe microscope lithography: a review, J Vac Sci Technol B 23, 877-894 (2005)] at locations with nanometer precision over a surface that is to be scanned. The probes or the surface being scanned are moved on a stage that is driven by a combination of motors including rotating electric motors and piezoelectric motors. These motors have considerable hysteresis and motion characteristics change due to aging of components and interfaces between components, both introducing sources of error in position control. The errors can be significant and if the probe is controlled to go back to a pre-defined starting position by following the motor's action in reverse, the probe can be off by distances too large compared to features that are of interest. Some typical examples of these features include quantum dots, nano-tubes and nano-scale transistors which have features in the 1-100 nm range. The aging and hysteresis errors affect the capability of the scanning probe to arrive at a given location and therefore when different samples are scanned, considerable time and effort is expended to find the location by scanning. This can be detrimental to scanning for defects in nano-manufacturing, increasing the time needed to achieve a high yield of nanoscale devices. In addition to scanning applications, the scanning probe technology can also be used to nano-fabricate or modify the surface under the probe by thermal [see e.g., P Vettiger, et al, The “Millipede”—more than one thousand tips for parallel and dense AFM data storage”, APMRC 2000, Tokyo Japan, MC1-01-MC 1-02; W P King, et al, “Atomic Force Microscope Cantilevers for Combined Thermomechanical Data Writing and Reading”, Appl Phys Lett 78, 1300-1302 (2001); R D Piner, J Zhu, F Xu, S Hong, C A Mirkin, “Dip Pen Nanolithography”, Science 283, 661-663 (1999)] or electronic [R Nemutudi, N J Curson, N J Appleyard, D A Ritchie, G A C Jones, “Modification of a shallow 2DEG by AFM lithography”, Microelectron. Eng. 967, 57-58 (2001)] effects. The need is usually to place nanostructures at precisely defined positions with precise distances between them, to realize predictable effects from device to device. For example, a typical task under tip-based nanofabrication (e.g., making pillars) using a probe is done by material deposition or removal under the probe. This requires the probe to be centered at the right x and y coordinates with nm accuracy. Furthermore, since the array placement accuracy might be needed over large distances (comparable to the size of wafers or die chips), getting very large arrays made with nm accuracy requires very high positioning accuracy. For example to achieve 1-nm accuracy over 1-cm requires a precision 1e-7 in placement. Such precision is not achievable in today's systems and therefore, time is laboriously spent searching for nanoscale features, limiting the throughput of nano-science and technology.
Current state-of-art in placement places the burden of probe location on the stage by incorporating optical interferometers across stage motion [see e.g., S Awtar, A H Slocum, “Target Block Alignment Error in XY Stage Metrology”, Precision Engineering 31, 185-187 (2007)]. Such interferometers count fringes and distance between fringes by calibrating motion. The interferometers work along a narrow axis, so that a large reflective block surface spanning the orthogonal width of the stage is required. This makes the stages typically bulky, and initial starting point is still needed to move to an exact place. The bulky stages and fringe counting can limit the speed at which scanning can be done, and especially the frequency at which the stage can accelerate. Furthermore, during fringe counting, error in measurement systems associated with the signal-to-noise ratio of fringe counting and stage hysteresis within each fringe ultimately adds to the measurement offsets, leading to substantial errors when applied to motion over several mille-meters. The precision in the distance measurement also depends directly on the flatness of the large reflective block, which is difficult to manufacture to such high precision in the first place and suffers from drifts (due to effects of e.g. thermal, acoustics, etc), especially comparing one end of the block to the other. Other approaches to stage motion measurement include capacitive and strain sensors, however these sensors also age, are limited in travel range, and have limited precision because the precision in measurement is directly equivalent to the precision of actual manufactured device (here, a pair of capacitive plates) [see e.g., P W Kolb, R S Decca, H D Drew, “Capacitive sensor for micropositioning in two dimensions”, Rev Sci Instrum 69, 310-312 (1998)].
In view of the foregoing, a need remains for an economical and reliable method and apparatus permitting high precision and high resolution wafer-scale nanofabrication.